The objective of this research is to gain insight into the mechanisms by which the obligately intracellular bacteria of the genus Chlamydia attach by and gain entry into host cells and survive within membrane-limited vacuoles. Specific aims include: 1. To study the various parameters of attachment of C. psittaci to L cells including the kinetics and reversibility of attachment, and the role of lipids, amino, and carboxyl groups of attachment. Attempts will be made to isolate the host-cell receptor for chlamydiae and understand the loss of infectivity of the parasite immediately upon ingestion. 2. To sutdy changes in the structure and properties of host-cell membrane following ingestion of chlamydiae and during the course of infection cycle. 3. To elucidate the mechanism by which the membrane which surrounds dividing chlamydiae grows. These studies should further the understanding of the general mechanisms of phagocytosis and survival of parasitic bacterial in host cells.